Semiconductor laser devices emit light having an optical power output level that is a non-linear function of the current flowing through the device. The transfer function, device current versus optical power output, of semiconductor laser devices is not stable and changes as a function of temperature and age. These devices must be controlled to maintain a desired optical power output level and insure that the electrical power dissipation does not exceed its' rated maximum and destroy the device. These devices are capable of high switching frequencies when maintained in a conducting state by supplying a bias current through the device. The bias current reduces the effective impedance of the device thereby increasing its switching frequency.
Previous circuits lacked in maintaining constant ON state and OFF state currents through the semiconductor laser device. This causes the laser beam to be unstable, optical power output is not at the desired level immediately following the transition from the OFF state to the ON state. This instability causes optical power density variations which introduce noise into the optical output signal.
Another problem with prior circuits is poor ability to adjust and maintain the operating points of the current driver to compensate for variations in the terminal characteristics of the semiconductor laser device due to temperature variations and age.
For example, a driver circuit for a laser diode is described in U.S. Pat. No. 4,761,659, issued on Aug. 2, 1988, to Negishi. This driver circuit compensates for variation of the relationship of laser diode current and laser diode optical power output due to temperature and age. The driver circuit compensates for these variations by using a diode in a reference circuit for controlling the power output of the laser diode. The reference voltage, produced by the reference circuit, varies with temperature in a manner similar to that of the laser diode which tends to maintain the laser diode optical power output reasonably constant. The driver circuit itself however limits high frequency operation since there is no bias current supplied to the laser diode during its' OFF state.
Another example is U.S. Pat. No. 4,709,369, issued on Nov. 24, 1987, to Howard. This patent describes a driver circuit that controls the power output of a laser diode. This driver circuit allows for a bias current through the laser diode for increased frequency of operation. The circuit adjusts the writing current and bias current levels with a single reference. This single reference is optimized to produce the desired power output in the writing state and the bias current is whatever results from the reference voltage through the other emitter resistor. This circuit does not maintain a constant contrast ratio between the ON and OFF states of the laser diode and can cause image shadow on sensitive imaging media as temperature varies.
It is therefore a general object of the invention to control the current flows within a semiconductor laser device to maintain a substantially constant optical power output while maintaining a high switching frequency of the semiconductor laser device.
It is a further object of the invention to automatically adjust the current flows within a semiconductor laser device to compensate for variations in the transfer characteristics of the device due to temperature and age in order to maintain the optical power output and contrast ratio substantially constant while maintaining the ability to switch the semiconductor laser device at a high frequency.